The leader of Syria's main political opposition group has said he is ready to negotiate with government officials whose hands are not "stained with blood" once the president, Bashar al-Assad, and his associates leave power.

Abdelbasset Seida, the head of the Syrian National Council, also told the pan-Arab Asharq al-Awsat newspaper in an interview published on Sunday that the resignation of the Syria peace envoy Kofi Annan may open the door for a new initiative to resolve the crisis.

His comments came as forces loyal to the regime pounded rebel positions in Syria's second city, Aleppo, on Sunday. The key battle there appeared to be looming, sources in the city indicated. The British foreign secretary, William Hague, warned that the coming days were likely to see an escalation of violence and bloodshed.

On the future of the ruling group, Seida said: "As far as we are concerned, the authorities have lost their credibility and legitimacy, and we have said this in Moscow bluntly: that dialogue with this regime is no longer possible.

"Bashar and his gang must leave and after that we will move to negotiate with other officials whose hands were not stained with Syrian blood and who were not involved in big corruption cases."

Annan resigned as the UN and Arab League envoy on Syria last week as a result of frustration at the world body's failure to act decisively to end 17 months of bloodshed.

On Friday, UN member states voted overwhelmingly to condemn the Syrian government for the violence at a special session of the general assembly. Syria's allies Russia and China opposed the non-binding resolution but were not able to use the veto they have used in the security council.

Seida welcomed Friday's vote: "We believe that the vote at the United Nations general assembly represents the start of a new initiative that may be coming in the near future."

He did not elaborate.

Reports from Aleppo – Syria's largest city and commercial hub – said that Assad's forces were using heavy artillery, planes and a helicopter gunship to bombard rebel positions in a battle that could determine the outcome of the uprising.

Hague warned that the regime's reliance on violence, rather than negotiations, in responding to challenges to Assad's rule, could lead to "a collapse in all authority" in the country.

Separately, the US secretary of state, Hillary Clinton, will travel to Istanbul this week to hold talks with the Turkish government on the crisis, a state department spokeswoman said on Sunday.

"Secretary Clinton goes to Istanbul for bilateral consultations with the Turkish government on Syria as well as to cover other timely issues," Victoria Nuland said in a statement sent to reporters during Clinton's visit to Malawi.

Concern is growing in Turkey about the growing influence in northern Syria of the Kurdish Democratic Union Party (PYD), a group linked to Kurdish separatists fighting Ankara.

Clinton's planned talks in Istanbul on 11 August will form part of renewed international efforts to tackle the escalating crisis in Syria.

Despite Seida's comments on Sunday and the vote in the UN general assembly on Friday, it is widely perceived that prospects of a negotiated solution have dimmed since Annan resigned.

Nuland also said that Clinton, whose current tour will take her to South Africa later on Sunday, will visit Nigeria, Ghana and Benin on her way to Istanbul.